Odd lot: A unit of trading in securities that is less than 100 shares. (See also "round lot.")

Offer price: See asked price.

Official statement: The document that provides key information regarding a municipal bond new issue.

Operating expense ratio (OER): The proportion of a mutual fund's average net assets required to pay annual expenses (operating expenses, management fees, etc.).

Open: The first price of a given security or commodity at the beginning of trading on any given day.

Open-end fund: A mutual fund with no limit to the number of shares that can be issued.

Open order: An order to buy or sell securities that has not yet been executed or canceled.

Option: A contract that allows an investor to sell or purchase an asset at a fixed price until a specific date. An option to purchase an asset is a call and an option to sell an asset is a put.

Over-the-counter (OTC): A highly sophisticated communications network on which dealers trade securities that are not listed on any exchange.